The study of human cells defective in repairing damaged DNA was extended, with the rationale that DNa-repair deficient cells are more susceptible to the adverse effects of carcinogens (cell killing, mutagenesis, sister chromatid exchange, and malignant transformation) than their repair-proficient counterparts. A group of human tumor (19) and SV40-transformed (7) strains deficient in the repair of 0-six-methylguanine (0-6-MeG, a modified DNA base made by certain methylating agents) was identified earlier in this project. Such strains were called Mer (-). Like SV40 and Rous sarcoma virus, adenovirus was found to produce Mer (-) strains. An activity was present in 8 Mer (+) strains but not in 11 Mer(-) strains that demethylated 0-6-MeG in DNA, thereby producing guanine and repairing this damaged base. After the reaction, the methyl group was found bound to a 22,000 M.W. protein, presumably the 0-6-MeG-DNA methyltransferase. Using methyl group transfer as a stoichiometric measure, Mer(+) strains were judged to contain on the average 60,000 methyltransferase molecules per cell. A group of 5 Mer(+) cell strains, found sensitive to cell killing by MNNG was termed Mer(+) Rem(-). This group was found able to repair approximately one-third as much 0-six-methylguanine as Mer(+) Rem(+) cell strains. To provide a basis for somatic cell genetic studies of DNA repair, hybrids between cells having the various DNA repair phenotypes were produced after preparing cells with selectable markers by transfection with plasmids. Primary fibroblasts and established cell lines from various strains of mice show differences in their ability to respond to agents that produce 0-6-MeG. Finally, studies with hydroxyurea indicate that human tumor cells and normal human fibroblasts depend upon ribonucleotide reductase(s) to different extents when repairing UV-damaged adenovirus.